Method of making large grain-sized superalloys

ABSTRACT

A process for making nickel-base superalloys possessing superior high-temperature properties which employs powder metallurgical techniques and includes the steps of densifying the powdered alloy into a blank approaching 100 percent theoretical density, cold working the blank at a controlled temperature, recrystallizing the cold-worked blank for a period of time sufficient to nucleate new grains and thereafter heat treating the recrystallized blank at a controlled temperature for a period of time sufficient to attain the desired magnitude of grain growth.

United States Patent Reichman et al.

[ Feb.l,]l972 [54] METHOD OF MAKING LARGE GRAIN- SIZED SUPERALLOYS [72]lnventors: Steven H. Reichman; John W. Smythe,

both of Ann Arbor, Mich.

Primary Examinerl-lyland Bizot Assistant ExaminerW W. StallardAttorney-Harness, Dickey & Pierce [57] ABSTRACT A process for makingnickel-base superalloys possessing superior high-temperature propertieswhich employs powder metallurgical techniques and includes the steps ofdensifying the powdered alloy into a blank approaching 100 percent [52]U.S.Cl. ..l48/11.5 F, 75/ 171 theoretical density d working the blank ata controlled CL temperature, recr stallizing the cold-worked blank for a581 m Id we in 148/115-75/05 m y l e a period of time sufficient tonucleate new grains and thereafter heat treating the recrystallizedblank at a controlled tempera- [56] Reemmes Cited ture for a period oftime sufficient to attain the desired mag- UNITED STATES PATENTS ofgrain gwwth- 3,524,744 8/1970 Parikh ..75/ l7] 7 Claims, 4 DrawingFigures Maw/f /2// Mia? Evy w? m @MEHE METHOD OF MAKING LARGEGRAIN-SIZED SUPERALLOYS BACKGROUND OF THE INVENTION Modern superalloysof the general types to which the present invention is applicablecontain large amounts of second-phase gamma-prime and complex carbidesin a gamma matrix which contribute significantly to theirhigh-temperature properties. The presence of these constituents,however, has made such alloys exceedingly difficult to form subsequentto casting. Additional problems are further introduced as a result ofthe tendency of such alloys to undergo segregation, which significantlydetracts from their high-temperature strength characteristics. Theelimination of such segregation is virtually impossible due to theextend of it.

The foregoing problems have been overcome by employing powdermetallurgical techniques for making bodies of such superalloys. Inaccordance with this technique, the superalloy is microcast or atomizedto a powder state and then consolidated in a substantially oxygen-freeenvironment to a blank of the desired size and configuration, which issubstantially free from segregation. A continuing problem experienced insuperalloy components made by such powder metallurgical techniques hasbeen the severe limitation in effecting any appreciable grain growth inthe resultant densified component. It is believed that such grain growthrestriction is in part attributable to oxides and other relativelyinsoluble impurities which are present on the surfaces of the powderparticles. Various precautions taken to reduce the presence of suchinsoluble impurities have not been successful since the problem inachieving such grain growth has been encountered even with powderedalloys containing as little as 30 parts per million (p.p.m.) oxygen.

In accordance with the process comprising the present invention, theproblem of effecting grain growth in densified powder components has nowbeen overcome providing for a metallurgical structure which is ofsuperior homogeneity and of superior physical properties at elevatedtemperatures than cast and wrought forms of the same superalloycompositions.

SUMMARY OF THE INVENTION The benefits of the present invention areachieved by an improved process for making large grain-sized nickel-basesuperalloys in which the alloy is initially microcast or otherwisesubdivided into a powder form of controlled size and is thereafterconfined and densified into a body or blank approaching substantially100 percent theoretical density. The dense body is subjected to coldworking at a temperature below the recrystallization temperature of thealloy and thereafter is recrystallized at a temperature between therecrystallization temperature and the solvus of the gammaprime phase fora period of time sufficient to nucleate new grains. The recrystallizedbody is thereafter heat treated at a temperature above the solvus of thegamma-prime phase and below the incipient melting temperature of thealloy for a period of time sufiicient to effect grain growth and theattainment of the desired ultimate grain size.

The nickel-based superalloys made in accordance with the processcomprising the present invention are characterized as being ofexceptionally large grain size and possessing superior tensile strengthand stress rupture life at elevated temperatures, that is, temperaturesin excess of about l,400 F. in comparison to similar-type alloysheretofore known.

Further advantages and benefits of the present invention will becomeapparent upon a reading of the description of the preferred embodimentstaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic flow sheetillustrating the sequence of steps in accordance with the preferredpractice of the process comprising the present invention;

FIG. 2 is a photomicrograph of a Kallings etched sample taken at amagnification of 500 times of the grain structure of a superalloy afterdensification from loose powder to a density corresponding substantiallyto percent theoretical densiy;

FIG. 3 is a photomicrograph of the same alloy shown in FIG. 2 at thesame magnification after being cold worked and subjected torecrystallization; and

FIG. 4 is a photomicrograph of the grain structure of a Kallings etchedtensile specimen taken at a magnification of 10 times prepared from thealloy shown in FIGS. 2 and 3 after heat treatment to effect graingrowth.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in detail to thedrawing, and as diagrammatically shown in FIG. 1, the process comprisingthe present invention consists of five basic steps which are performedin the same sequence as illustrated in the flow sheet. As shown, anickel-based superalloy of the desired composition is initiallycomminuted or microcast so as to form a powder of the desiredconfiguration and particle size which thereafter is confined anddensified, forming a body or blank having a density approaching a 100percent theoretical density. The resultant blank is thereafter coldworked, that is, subjected to deformation at a temperature below therecrystallization temperature of the alloy, followed by arecrystallization step in which nucleation of new grain occurs.Thereafter, the recrystallized blank is subjected to a heat treatment ata controlled temperature, during which a growth in the grain size iseffected and by proper control, can be increased up to almost a singlecrystal structure.

The provision of the nickel-based alloy in the form of a metallic powderin which each of the powder particles is of substantially the samenominal composition can be achieved by a variety of techniques, of whichmicrocasting, such as achieved by atomization of a melt of the alloy,constitutes the most convenient and preferred technique. Themicrocasting of the molten alloy can be achieved, for example, by anatomization process employing an atomization nozzle and technique asdescribed in US. Pat. No. 3,253,783, which is assigned to the sameassignee as the present invention and is incorporated herein byreference.

Due to the deleterious effects of oxygen and oxides of the metalscomprising the alloy, the atomization of the superalloy and thecollection of the powder particles is achieved under conditions wherebyoxygen and oxygen-containing substances, including water, are notpermitted to contact the powder particles for any appreciable time tominimize oxidation and/or oxygen entrapment. The degree of precautionsrequired to prevent oxidation of the superalloy during the atomizationprocess is dependent to a large extent on the specific alloyingconstituents present in the alloy. For example, the presence of aluminumand titanium require particular precautions due to their susceptibilityto oxidation attack at the high temperatures encountered in conventionalmicrocasting techniques. Under such conditions, it is conventional toeffect microcasting in the presence of inert atmospheres such as argonor helium, which are substantially moisture free. Commercially availableargon containing minimal amounts of conventional impurities has beenfound particularly satisfactory for providing a nonoxidizing,substantially dry inert atmosphere for microcasting such superalloys. Inaccordance with conventional practice, the interior of the equipment tobe employed is initially evacuated and thereafter back-flooded with thesubstantially dry, nonoxidizing atmosphere prior to initiation of theatomization of the melt. Regardless of the specific technique employedfor forming the powder, the oxygen content of the powder as finallydensified is preferably controlled to a level of less than about I00p.p.m.

In accordance with conventional atomization or microcasting procedures,the superalloy is transformed into a metallic powder in which theparticles preferably are of a generally spherical configuration andwherein each powder particle is of substantially the same or similaralloy chemistry. The metallic powder is thereafter recovered and issubjected to a screening operation so as to segregate the powderparticles which are suitable for forming the densified body or billet ofsuperalloy. 5 Conventionally, particles of a size less than about 60mesh United States Standard Sieve Size (250 microns) can besatisfactorily employed down to a particle size as small as about 1micron. Particularly satisfactory results are obtained when the powderparticles range from about 100 mesh (150 microns) to about 10 microns,and wherein the particles are further randomly distributed over theaforementioned range. This provides for optimum packing density of thefree-flowing powder, facilitating subsequent densification thereof.

The resultant superalloy powder, having the desired composition andparticle size, is thereafter confined and densified at elevatedtemperatures so as to form a body or billet approaching 100 percenttheoretical density. The densification of the metallic powder can beachieved by any one of the variety of techniques well known in the art,including extrusion, hot upsetting, vacuum die pressing, hot isostaticcompaction, explosive compaction, etc. The densification process ispreferably done at an elevated temperature to facilitate a bond of thepowder particles and to facilitate compaction and- 5 deformation thereofinto a billet approaching substantially 100 percent theoretical density.For most nickel-base superalloys, preheat temperatures ranging from1,900 F. up to about 2,500 F. can be satisfactorily employed. Thespecific temperature used within the aforementioned range is dictated bythat temperature approaching the solidus or just below the incipientmelting point of the powder particles. The aforementioned explosivecompaction technique in which the powder is subjected to violentdensification is usually done without any appreciable preheat. In theextrusion and hot upsetting compaction techniques, it is conventional toconfine the powder within a suitable container which is evacuated andsubsequently sealed. Optimum packing of the interior of such containerswith the loose powder can be achieved by subjecting the containers tosonic or supersonic frequencies wherein packing densities ranging fromabout percent to about 70 percent of a theoretical 100 percent densitycan be attained. It is also contemplated that the loose powder particlescan be confined in the cavity of a die, subjected to vacuum andcompacted so as to make a preform approaching 85-90 percent theoreticaldensity. Such a preform can also be attained by compacting the powder invacuum and sintering it at an elevated temperature, forming aself-sustaining body or billet which subsequently can be subjected tofurther compaction to attain substantially 100 percent density.

Of the foregoing compaction techniques, hot extrusion of the powderwhile contained within an elongated deformable container has been foundconvenient and satisfactory for producing the improved superalloy inelongated rod form. Such containers may comprise any metal havingsufficient ductility to enable their deformation by extrusion atelevated temperatures without rupture of the sidewalls, therebymaintaining the sealed integrity of the powder particles therein.Typical of such ductile metals which are compatible with the 60superalloy powder and which can be satisfactorily employed for thepractice of the present invention are various of the socalledconventional stainless steels such as AlSl-type 304 or an M81 1010 mildsteel.

At the completion of the compaction or densification operation, theresultant densified billet is allowed to cool and is thereafter coldworked by subjecting it to a mechanical deformation, such as by passingit between a pair of rolls or by subjecting it to a further extrusionoperation. The cold working of the densified billet can be achieved inone or more successive passes to impart he desired degree of cold workto the billet, which is dictated by that amount necessary to provide fora substantially complete recrystallization of the alloy at the specifictemperature used during the following recrystallization step. For mostnickel-base superalloys, it has been found that the magnitude of coldworking expressed in terms of percentage reduction of thecross-sectional area of the densified body or billet during such coldworking can range from only several percent up to about 50 percent ormore. The maximum degree of cold working imparted to the densifiedbillet is dictated by practical considerations, including equipmentlimitations and time. Usually, 50 percent reductions in crosssectionalarea in one pass have been found satisfactory and cross-sectional areareductions or the equivalent cold working in a range of about 30 percentto about 50 percent at moderate temperatures ranging from about l,000 F.to about l,700 F. constitutes a preferred practice.

During the cold-working step, the densified blank or billet ispreferably heated to facilitate deformation thereof and as previouslyindicated, can be heated to moderate temperatures which approach but arebelow the recrystallization temperature of the specific alloy. For mostnickel-based superalloys of the type to which the process comprising thepresent invention is applicable, the recrystallization temperaturegenerally is in the range of from about 1,700 F. to about 2,l00 F. Inview of this, it is preferred to heat the densified billet to atemperature of from about 1,000 F. to about l,700 F. during such coldreduction.

For the purpose of the present invention, the terminologyrecrystallization temperature, as employed in the specification andsubjoined claims, is defined as that temperature above which anucleation and growth of new strain-free grains occurs accompanied byconsumption of the cold-worked matrix as a result of the growth of suchgrains.

The resultant densified and cold-worked billet is thereafter subjectedto recrystallization at a temperature above the minimumrecrystallization temperature but below the gammaprime solvustemperature. The gamma-prime solvus temperature, as herein used, isdefined as the temperature at or above which the gamma-prime phasedissolves in the gamma phase matrix. The gamma-prime phase in turn isdefined as a variety of intermetallic compounds which are generallyexpressed by the formula Ni,,(X,Y,Z) in which X, Y and Z represent, forexample, aluminum, titanium, cobalt, etc., and wherein a" and b" areintegers. These intermetallic compounds at temperatures below thegamma-prime solvus temperature are dispersed throughout the gamma matrixand act as a strengthening agent.

In accordance with the preceding definitions, recrystallization of thecold-worked and densified billet is achieved at a temperature generallyranging from about l,700 F. up to about 2,l00 F. for a period of timesufficient to effect a nucleation of new strain-free grains in thecold-worked billet. Recrystallization is continued for a period of timesufficient to effect substantially full recrystallization of the billet,which, for most nickel-based superalloys which are cold worked in anamount ranging from about 10 percent to about 50 percent in terms ofreduction of cross-sectional area or the equivalent thereof atrecrystallization temperatures of from l,700 F. up to about 2,100 F.,requires about 2 to about 12 hours. It will be noted that therecrystallization of a cold-worked billet can be performed at any timeafter the cold working and similarly, the heat-treating step can beperformed at any time after the recrystallization step. The absence ofany criticality in time with respect to the performance of the severalprocess steps provides further advantages in connection with theversatility and processing flexibility afforded.

At the completion of the recrystallization step, the densified,cold-worked and recrystallized billet is subjected to a heat treatmentin which grain growth occurs. The heat-treating operation is carried outby heating the recrystallized billet to a temperature above thegamma-prime solution or solvus temperature and below the incipientmelting point of the gamma matrix. The incipient melting point of thegamma matrix for nickel-based superalloys of the general type to whichthe process is applicable conventionally ranges from about 2,200 F. upto about 2,500 F. The duration of heat treatment can be varied so as toprovide the desired degree of from about 2,l0O F. to about 2,400 F. fornickel-based superalloys of the general type evaluated have been foundsatisfactory to produce a resultant microstructure in which the grainsize is approximately one-eighth inch in diameter. It is feasible, bycontinuing the heat treatment of the billet over prolonged periods oftime, to effect further increases in grain size until ultimately abillet of a single grain crystal is attained.

It will be apparent from the foregoing that it is now feasible,employing powder metallurgical practices, to form billets and componentscomposed of nickel-based superalloys which are of a relatively largegrain structure and possess superior hightemperature physical propertiesin comparison to the same or similar superalloys in a cast and/orwrought form. The benefits of the process comprising the presentinvention are achieved with any one of a variety of well-knownsuperalloys which are nickel based, that is, in which the major alloyingconstituent is nickel. Typical of the various nickel-based alloys whichare presently known and which can be processed in accordance with thepresent invention are the compositions as set forth in table ll. It willbe understood that the enumerated superalloy compositions are providedfor illustrative purposes and are not intended as being restrictive ofother suitable nickel-based alloy compositions that can besatisfactorily processed to achieve the benefits of the presentinvention.

3. It is apparent that the cold-worked and recrystallized grainstructure of the billet as shown in FIG. 3 evidences a finerecrystallized grain structure.

Following the recrystallization step, the billet was subjected to heattreatment at a temperature of 2,150 F. for a period of about 72 hours.The heat treatment temperature employed is above the gamma-prime solvustemperature but below the incipient melting temperature of this alloy.The large grain structure attained as a result of the heat treatmentstep is clearly evident in the photomicrograph comprising FIG. 4 of thedrawings which comprises a Kalling's etched micrograph of a tensilespecimen prepared from the billet and photographed at a magnification of10 times.

In comparison, a control specimen prepared from the same powder andsubjected to the same compaction by extrusion followed byrecrystallization and heat treatment, but omitting the cold-workingstep. did not evidence any appreciable grain growth characteristics andpossessed high temperature physical properties substantially inferior tothat of the specimen as evidenced by the microstructure shown in FIG. 4.Comparative room and elevated temperature tests of the tensileproperties of the alloy prepared in accordance with the processcomprising the present invention and the same alloy in a cast-andwroughtcondition revealed the alloy made in accordance with the processcomprising the present invention to be at least as good, and in mostcases, superior to that of the prior art struc- [Percent by weight]Alloy C Cr Al Ti Mo W Go Cl) :3 Zr Other Ni Nlrnuulc 75... 0.12 20 0.5Bztluncv. Nimtmlc 80A 0. 08 20 1,5 2.4 H l)(j Nimonic 10, 0.10 20 1. (l2. 4 Do. Niuumictlfi. 0.12 20 2.0 3.0 Do. Nlmrmlr. 100.. 0.20 II 5.0 1.35.0 [)o. Wuspnloy. 0.08 10 1.3 3. 0 4. 4 0. 008 0. 08 Do. lhllmul. 7000.10 15 4.3 3.5 5.2 0.03 1m. Hum! 41 0.00 10 1.5 3.1 10.0 0.005 no. IN-l0ll (011st). 0.18 10 5. 5 5.0 3. 0 15. 0.015 0. 05 Du. m uumun 0.150.0 5.0 2.0 1.0 0 015 0.05 1m. ll-llllll] (c1tSl.) 0.11 8.0 0.0 1.0 0.010. 0. 015 0. 07 4.3 T0,... 110. moo-713 (c11st).... 0.14 13.0 6.0 0.754.5 2.4 0.01 0.1 110. M-; 2 0.15 10.0 1.0' 2.5 0.8 10.0 1 0.005 5.0F0(mux.). Do.

In order to further illustrate the process comprising the tures. Inaddition, stress rupture properties, a property parpresent invention,the following typical examples are proticularly important in alloyssubjected to high temperature vided. It will be understood that theexamples are furnished stress applications, were measured at atemperature of l,850 for illustrative purposes and are not intended tobe limiting of F. and at a stress of 20,000 psi. for the alloycomprising the the scope of the invention as herein described anddefined in present invention and identical alloy compositions of thecastthe subjoined claims. and-wrought type heretofore known. The alloyprocessed in accordance with he present invention had a stress rupturelife EXAMPLEI j to failure of 196 hours, whereas conventional cast-and-A millfel'based sljlperanoy correspondmg the nominal wrought U-700 alloyof the same composition had a life of composition of Udrmet 700, as setforth in table 1, was only 10 hours under these same conditionsmicrocast into spherical powder particles and were screened while itwill be apparent that the description f the Pmviding f 'f y Filed Powderranging from 10 mlcmns preferred embodiments of the present invention iswell calcu- P to micro! S1Ze- The Q S Powder was lated to provide theadvantages and benefits of the process fined an elongated cylmflncalcomalllel' composed Ofa "111d comprising the present invention, it willbe appreciated that Steel and compacted harem y Subjectmg the comalnel'0 the process is susceptible to variation, modification and p rsomc vlra n Th Contalner was q n ly eVaCU- change without departing from thespirit of the invention. ated and sealed by welding and thereafter wasextruded to a wh t i l i d i fully dense rod, while heated to atemperature of l,950 F. 1, The ethod offorming a dense mass of anickel-based su- The microstructure of the resultant densified billet isillusperalloy which comprises the steps of confining and densifyingtrated in FIG. 2. The resultant extruded rod thereafter was a powder ofsaid superalloy into a billet, cold working said bilpreheated to l,700F., which is approximately 200 F. below let by effecting deformationthereof at a temperature below its recrystallization temperature. Atthis preheat temperature, the recrystallization temperature of thealloy, recrystallizing the billet was cold worked by passing it througha pair of rolls, the cold-worked said billet by heating; it to atemperature effecting approximately a 50 percent reduction incross-secabove its recrystallization temperature and below thegammational area in one pass. The resultant cold-worked billet was primesolvus temperature for a period of time sufficient to efthereafterrecrystallized for a period of 2% hours at a temperafeet nucleation ofnew grains, and thereafter heat treating the ture at 2,100 R, which is atemperature above the recrystalrecrystallized said billet at atemperature above the gammalization temperature but below thegamma-prime solvus temprime solvus temperature and below the incipientmelting perature for this alloy. The resultant recrystallized structureof point of the gamma matrix for a period of time sufficient to efthecold worked and recrystallized billet is illustrated in FIG. feet growthof the grain to the desired size.

2. The method as defined in claim 1, wherein said powder is of aparticle size ranging from about 60 mesh to about 1 micron and containsless than about 100 p.p.m. oxygen.

3. The method as defined in claim 1, wherein said billet formed by saidconfining and densifying is substantially of a 100 percent theoreticaldensity.

4. The process as defined in claim 1, wherein said cold working isperformed so as to provide a degree of cold working to said billetequivalent to that resulting from a reduction in its cross sectionalarea of from several percent up to about 50 percent.

5. The method as defined in claim 1, wherein said cold working isperformed on said billet which is preheated to a temperature rangingfrom about l,O00 F. to about l,700 F. in a manner to impart a workingthereof equivalent to that resulting from about a 30 percent to about a50 percent red uction in its cross-sectional area.

6. The method as defined in claim 1, wherein said recrystallizing thecold-worked said billet is accomplished for a period of time rangingfrom about 2 hours up to about 12 hours at a temperature of about 1,700F. to about 2,100" F.

7. The method as defined in claim 1, wherein the recrystallizationtemperature of said superalloy ranges from about l,700 F. to about2,100? F.

2. The method as defined in claim 1, wherein said powder is of aparticle size ranging from about 60 mesh to about 1 micron and containsless than about 100 p.p.m. oxygen.
 3. The method as defined in claim 1,wherein said billet formed by said confining and densifying issubstantially of a 100 percent theoretical density.
 4. The process asdefined in claim 1, wherein said cold working is performed so as toprovide a degree of cold working to said billet equivalent to thatresulting from a reduction in its cross sectional area of from severalpercent up to about 50 percent.
 5. The method as defined in claim 1,wherein said cold working is performed on said billet which is preheatedto a temperature ranging from about 1,000* F. to about 1,700* F. in amanner to impart a working thereof equivalent to that resulting fromabout a 30 percent to about a 50 percent reduction in itscross-sectional area.
 6. The method as defined in claim 1, wherein saidrecrystallizing the cold-worked said billet is accomplished for a periodof time ranging from about 2 hours up to about 12 hours at a temperatureof about 1,700* F. to about 2,100* F.
 7. The method as defined in claim1, wherein the recrystallization temperature of said superalloy rangesfrom about 1,700* F. to about 2,100* F.